I was about to ask what made you think Stack Exchange uses Highcharts - then I did a search in the HTML source...
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Andrew BarberApr 21 '14 at 14:00

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This question was also asked on Meta.StackExchange; please don't do that. Pick one site, it'll get migrated if it's not the best place. (Discussions might make sense in multiple venues, this doesn't.)
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MatApr 21 '14 at 15:39